The present invention relates to inboard type magnetic system for an electro-dynamic transducer and particularly pertains to a loud-speaker.
The magnets used in the magnetic system of ordinary load-speakers are mostly Alnico magnets and ferrite magnets, the former being used mainly in the inboard type magnetic systems of medium and small size loud-speakers, and the latter mainly in the outboard type magnetic systems of large loud-speakers.
Of late, it is the trend that increasingly larger loud-speakers are being employed for improvement of the tone quality in portable sonic devices including radios, cassette tape recorders, etc., and as the loud-speaker grows larger, the device as a whole is enlarged, especially being given an increased height or thickness, resulting in reduced portability of such portable devices. For this reason, the reduction in height or thickness of loud-speakers is particularly desired. The loud-speaker must be so arranged that the magnetic flux leaking from the loud-speaker will not affect Braun tubes, medium frequency transformers, etc., in color television and portable radio sets, etc. The outboard type loud-speakers in which ferrite magnets are used cannot be adopted for such uses because of the very large magnetic flux leakage, and for this reason, usually, loud-speakers in which the inboard type magnetic systems are used are employed. In the case of the inboard type systems, however, conventional products, because of the substantial magnetic flux leakage, must have the parts arranged with some spacing thereof, and accordingly, the size-reduction for such systems is believed to have already approached the ultimate limit.
Heretofore, it has been commonly known that the increase in the residual flux density Br and in the gap flux density Bg which works effectively on the voice coil is a sine qua non and thus an Alnico 5DG magnet is employed for the magnetic systems of loud-speakers.
FIG. 1 is a section of an inboard type magnetic system in which a conventional Alnico magnet is employed, in which 1 denotes the Alnico 5DG magnet; 2, the mild steel yoke; 3, the mild steel center pole, and 4, the voice coil, with the arrows indicating the magnetic fluxes. .phi.1 designates the gap magnetic flux which effectively works on the voice coil; .phi.2, the magnetic flux leakage between the top of the center pole 3 and the top of the yoke; .phi.3 the magnetic flux leakage between the outer circumference of the magnet 1 and the center pole 3 and the inner circumference of the yoke 2; and .phi.4, the magnetic flux leakage from the back of the yoke.
As for ferrite magnets, because the Br thereof is so small, magnets with large areas become necessary, making it difficult to adopt them for use in the inboard type structure; as a consequence, the flux leakage is very large; the utilization efficiency of magnetic fluxes is lower than in the inboard type; the area and diameter required for magnetic devices are larger, and moreover, the yoke is required to be very thick.
In addition to the foregoing, for answering the recent demand for the preservation of resources and energy, that is, the demand for reduction in the current and power consumption, many attempts have been made to improve the efficiency of loud-speakers. Some methods for reducing the gap and increasing Bg by way of improving the voice coil winding method have been contemplated, but these methods have attained an improvement in efficiency of at most about 5%, and the alternative structures of the magnetic devices have already been exhaustively developed.
Furthermore, in an attempt to reduce the leakage of magnetic fluxes toward the back of the loud-speaker, there has been contemplated a method in which a hollow layer is formed in the yoke on the back of the magnetic device, to increase the magnetic resistance, thereby reducing the leakage backward of magnetic fluxes, but in this method, the magnetic system will inevitably be made higher or thicker.
The Alnico base magnets have a large permeance coefficient P at their optimum operating points, that of Alnico 5DG being P = 17 .about. 18 G/Oe. Accordingly, if the magnetic energy is to be effectively utilized, the ratio of the height to the diameter of the magnet L/D must increase, necessitating the magnetic system to be higher.
Moreover, according to the conventional design approach, Bg is increased by focusing the magnetic fluxes by use of the center pole. By this, the magnetic system is made higher and higher, and its weight becomes greater.